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Low Mag Video Lenses
Understanding Focal Length
and F/Number
Video lenses can be classified
into three categories according to
focal length: standard, wide angle
and telephoto. Focal length is the
distance between the camera sen-
sor and the center of the lens. The
greater the focal length, the larger
the image will appear. Therefore,
the greater the focal length, the
more the lens becomes telephoto
in application.
Standard Lens
A standard lens doesn’t change the
size of the object being viewed.
Wide Angle Lens
A wide angle lens provides a
wider field of view and therefore a
smaller image of the object being
viewed than a standard lens.
Telephoto Lens
A telephoto lens produces a larger
image of a distant object. The
longer the focal length, the larger
the object will appear.
The f/number is an indication of
the brightness of the lens. It is the
measurement of the ratio between
the focal length and the diameter
of the entrance pupil (where the
light enters the lens). The f/number
is directly proportional to the focal
length and inversely proportional
to the effective diameter of the
lens. It determines the amount of
light reaching the camera sensor.
The smaller the value, the larger
the opening and the brighter the
image produced by the lens.
How to Determine the Lens Focal Length Required
To choose the proper lens for a particular application, the fol-
lowing factors must be considered:
• Field of View - The size of the area to be imaged.
• Working Distance (WD) - Distance from the camera lens to
the object or area under surveillance.
• CCD - The size of the camera’s image sensor device.
• You must be consistent. If you are measuring the width of
your object, then use the horizontal CCD specifications, etc.
If you are working in inches, then do your calculations in
inches and convert to millimeters at the end.
FL = CCD x WD
FOV
Consider this example: You
have a 1/3” C-mount CCD cam-
era (4.8 mm horizontal). There
is a 12” (305 mm) distance be-
tween the object and the front
of the lens. The field of view,
or object size, is 2.5” (64 mm).
The conversion factor is 1” =
25.4 mm (round up).
Calculation in mm:
FL = 4.8 mm x 305 mm / 64 mm
FL = 1464 mm / 64 mm
FL =
23 mm Lens Required
Calculation in inches:
FL = 0.19” x 12” / 2.5”
FL = 2.28” / 2.5”
FL = 0.912” x 25.4 mm/inch
FL =
23 mm Lens Required
Please do not
confuse working
distance with object to
image distance. Work-
ing distance is mea-
sured from the front of
the lens to the object
being viewed. Object
to image distance is
measured from the CCD
sensor to the object.
To calculate the lens
focal length required,
you must use working
distance.
Distance Diagram
